Members of Amnesty International Mexico protest outside the Turkish embassy in Mexico in support of activists, including the director of Amnesty Turkey, who are on trial in Turkey for promoting "chaos in society"

Turkish prosecutors Sunday demanded up to 15 years jail on charges of backing terror groups for 11 activists, most of whom were detained in a raid on a workshop led by Amnesty International.

Their case has sparked international alarm, particularly from Germany, and amplified fears of declining freedom of expression under President Recep Tayyip Erdogan.

The accused, who include the director of Amnesty Turkey Idil Eser, several prominent Turkish rights activists and one German citizen and one Swedish citizen, have been charged with "giving help to an armed terror group", the Dogan news agency said.

Prosecutors have asked for jail sentences of 7.5 to 15 years, Dogan said, without specifying the request for each suspect.

In the indictment, the activists are accused of seeking to create "chaos in society" similar to the anti-government protests that rocked Turkey in the summer of 2013.

Ten activists were detained in July in a raid by police on a workshop session of human rights activists held on the island of Buyukada, a popular getaway spot off Istanbul.

A month before, Amnesty International's Turkey chair, Taner Kilic, was remanded in custody on charges of links to the alleged mastermind of the July 15 failed coup, the Muslim preacher Fethullah Gulen. Gulen rejects the accusations.

His case has now been merged with that of the other ten activists and faces even more serious charges of "membership" of an armed terror group.

State-run Anadolu news agency said Kilic faced up to 15 years in jail and the other suspects from between five years to 10 years.

'Absolutely incomprehensible'

The two foreigners -- German Peter Steudtner and Swede Ali Gharavi -- were leading the digital information workshop on Buyukada and have been under arrest ever since the July raid.

A member of Amnesty International Mexico holds a sign reading "Freedom" during a protest outside the Turkish embassy in Mexico City on July 31, 2017

(AFP)

Steudtner's detention has stoked tensions in particular with Berlin, whose relations with Ankara have plunged in recent months.

Germany sharply criticised the mass crackdown that followed the failed coup which has included the detention of several German citizens.

German Foreign Minister Sigmar Gabriel said he saw "with great concern" charges being laid against Steudtner and the other activists.

He said the terror charges against Steudtner were "absolutely incomprehensible" and the threat of a jail term "unacceptable", adding that Berlin had immediately contacted Ankara.

"We continue to do everything we can to bring the imprisoned German citizens, including Peter Steudtner, back to Germany," he said.

Amnesty describes Gharavi as an IT strategy consultant and Steudtner as a "non-violence and well-being trainer".

Eight of the suspects -- including both foreigners, Eser and Kilic -- are being held in jail ahead of trial and three others free but still charged. It is not yet clear when the trial will start.

Those charged are accused of having links with various organisations outlawed by Turkey as terror groups including the Kurdistan Workers Party (PKK), the group of Gulen and the far-left Revolutionary People's Liberation Party-Front (DHKP-C).

It said Eser was accused of possessing material of the DHKP-C while Kilic was formally charged with being a member of Gulen's group and using the messaging system Bylock allegedly employed by his supporters.